Energy Storage in Lithium Air Batteries: Electrode Design, Characterization and Performance Evaluation Under Different Operation Conditions
The capacity of a lithium battery system can be enhanced remarkably by combining Li as anode directly with oxygen as cathode active material in a Li/oxygen (or lithium/ air) battery [1, 2].
The specific energy of lithium/air batteries is an order of magnitude larger than that achievable using conventional lithium or lithium ion batteries . At a nominal potential of about 3V, the free energy for the reaction of Li with oxygen, forming Li2O2 (2Li + O2 ↔ Li2O2), is over 11 kWh kg-1  far exceeding Li-ion battery chemistry that has a theoretical specific energy of about 400 Wh kg-1. Although the large free energy for the reaction of Li with O2 has attracted the interest of battery researchers for decades, as yet, commercialization of the Li/air cells has been mired by several problems such as their large capacity loss during cycling and poor cycleability .
The cathode electrode in a lithium/oxygen battery is a carbon electrode having a porous structure in which several electrochemical and transport processes occur simultaneously. The cell performance strongly depends on the morphology and structure of the carbon. Therefore the main challenge in this issue is the design and development of new electrodes to improve the kinetics of the air cathode and enhance the capacity; energy and power densities; and the stability of the energy delivered by these systems.
In the present work nanostructured cathode electrodes based on R-F carbon aerogels are prepared and the role of porous structure of the carbon on the performance of the cathode electrode is examined by galvanostatic charge/discharge and electrochemical impedance spectroscopy measurements in a Li/O2 cell. It is shown that the discharge voltage of the cell depends on the morphology of the carbon used in the oxygen electrode and a combined effect of pore volume, pore size and surface area of carbon affects its storage capacity.
Several electrochemical parameters are investigated to optimize the state of the charge conditions of the Li/O2 batteries. The results show that the effective capacity of the cell drops with increase in the discharge rate because of the increased polarization of cathode. However the cell’s cycleability improves with increasing the rate of discharge probably due to the ease of stripping the Li2O2 film formed on the electrode surface reversibly at higher rates, compared with the incomplete removal of discharge products formed within the pores at lower rates.
The performance of the cell discharged at different cut off voltages showed that decreasing the depth of discharge decreases the rate of capacity fade and improves the cell cycleability.
Study of the cell performance at different charge taper voltages showed that both cell’s capacity and cycleability improve with increasing charge taper voltage for charge potentials up to 4.45 V. For charge potentials above 4.45 V, the cell performance deteriorates with increase in the charge taper voltage significantly, probably due to the decomposition of the electrolyte at higher charge potentials.
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